The proposed study is designed to define and characterize some of the intrinsic neuronal interactions occurring in the brainstem during alterations in gastrointestinal activity. The characterization will focus on the response of brainstem neural mechanisms during physiological alterations known to occur in the stomach (e.g. distention, changes in tension accompanying contractions). Extracellular recordings will be made from single neurons responding to gastric distention localized within brainstem structures of anesthetized cats. The specific aims to be achieved from this data are (1) to characterize the response of brainstem neurons to pressure-volume relationships in the stomach, (2) topographic mapping to localize and characterize vago-vagal reflex mechanisms in the brainstem, (3) to determine the peripheral and central effects of certain drugs, neurotransmitters and hormones upon vago-vagal brainstem interactions and (4) to characterize the source of the brainstem activity in response to alterations in gastric tension and electrical stimulation of gastric vagal fibers. The response of brainstem neurons to pressure-volume relationships will be examined during phasic distention of the stomach and changes in wall tension. The source of the brainstem activity will be evaluated and their location will be marked to topographically localize these cell populations. Drugs, neurotransmitters and hormones will be administered peripherally and centrally to characterize the role and nature of the sites of action. Three physiological responsibilities have been suggested for the proximal stomach: (1) receptive relaxation, (2) accommodation and (3) regulation of intragastric pressure. The role of distention and the mechanisms which regulate gastric distention are of great functional importance. In addition, the feelings of fullness and satiety are associated with the content of the proximal stomach. We have developed a model system and recorded data from 70 brainstem units which responded to gastric distention. This model provides "a new window" through which to view and characterize vago-vagal brainstem reflex mechanisms which have thus far eluded understanding.